BPS2026 – Poly-E and poly-D peptides respond differently to pH shifts and divalent ion concentrations

One of the characteristics of intrinsically disordered proteins/regions (IDPs or IDRs) is their high charge density and low sequence complexity, which helps them to maintain their highly flexible state. A unique feature of a number of acidic IDPs is a high percentage of glutamic acid but little to no aspartic acid. One IDR with this characteristic is titin’s PEVK region. This region has a number of glutamate-rich sequences (called poly-E motifs) that contain repeats of 2-3 hydrophobic residues followed by 3-4 glutamic acids (E), but there are no aspartic acids in these sequences. We hypothesized that the chemical characteristics of glutamic acid facilitate structural characteristics that aspartic acid is not able to mimic. To test this hypothesis, we synthesized 28 amino acid peptides based on the PEVK sequence that contained either poly-E or poly-D clusters and tested their structural characteristics using a combination of HPLC, circular dichroism (CD), and NMR diffusion. Both peptides exhibit more collapsed structures at lower pH, though the poly-D peptide appears to be more collapsed than the poly-E peptide. They both also show conformational changes in the presence of divalent metals, with the poly-D peptide showing similar sensitivity to both magnesium and calcium while the poly-E peptide was more sensitive to calcium. We have also attached these peptides to gold nanoparticles and have observed similar behaviors. Overall, our results support the hypothesis that the subtle differences in the chemical nature of glutamate and aspartate result in functional differences in peptides rich with one type of amino acid.